CHAPTER II
LITERATURE SURVEY
2.1 Carbon Black
Carbon black is a generic term for an important family of products
used principally for the reinforcement of rubber, as a black pigment, and for
its electrically conductive properties. It is a fluffy powder of extreme fineness
and high surface area, composed essentially of elemental carbon. Carbon black
manufacturing plants are strategically located worldwide in order to supply the
rubber tire industry consuming 70% of production. About 20% is used for
other rubber products and 10% is used for special non-rubber applications.
Carbon blacks differ from other forms of bulk carbon such as
diamond, graphite, cokes and charcoal in that they are particulate, composed
of aggregates having complex configurations, quasi-graphitic structure and
colloidal dimensions. They differ from other bulk carbons in having their
origin in the vapor phase through the thermal decomposition and the partial
combustion of hydrocarbons.
Carbon black is a product of a process
incorporating the latest engineering technology and process controls.
Its
purity differentiates it from soots that are impure by-products from the
combustion of coal and oils and from use of diesel fuels. Carbon black are
essentially free of the inorganic contaminants and extractable organic residues
characteristic of most forms of soot.
A number of processes have been used to produce carbon black
including the oil-furnace, impingement (channel), lampblack, and the thermal
decomposition of natural gas and acetylene.
These processes produce
different grades of carbon and are referred to by the process by which they are
made, e.g., oil-furnace black, lampblack, thermal black, acetylene black and
channel-type impingement black. A small amount of by-product carbon from
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manufacturing synthesis gas from liquid hydrocarbons has been found
applications in electrically conductive compositions. The different grades
from the various processes have certain unique characteristics, but it is now
possible to produce reasonable approximations of most of these grades by the
oil-furnace process. Since over 95% of the total output of carbon black is
produced by the oil-furnace process, this article emphasizes this process.
2.1.1 Physical Structure of Carbon Black
The arrangement of carbon atoms in carbon black has been well
established by x-ray diffraction method. Carbon black can have a degenerated
graphitic crystallite structure. Whereas graphite has three-dimensional order,
as seen in the model structure of Figure 2.1, carbon black has two-dimensional
order. The x-ray data indicated that carbon black consists of well-developed
graphite structure platelets stacked roughly parallel to one another but random
in orientation with respect to adjacent layers.
2.1.2 Chemical Composition
Oil-furnace blacks used by the rubber industry contain over 97%
elemental carbon. Thermal and
acetylene black consist of over 99% carbon. The ultimate
analysis of rubber-grade blacks is shown in Table 2.1. The elements other than
carbon in furnace black are hydrogen, oxygen, and sulfur, and there are
mineral oxides and salts and traces of adsorbed hydrocarbons. The oxygen
content is located on the surface of the aggregates as CxOy complexes. The
hydrogen and sulfur are distributed on the surface and the interior of the
aggregates. Some special blacks used for pigment purposes contain larger
quantities of oxygen than normal furnace blacks. These blacks are made by
oxidation in a separate process step using nitric acid, ozone, air and other
oxidizing agents. They contain from 2 to 6% oxygen. Oxidation improves
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dispersion and flow characteristics in pigment vehicle systems such as
lithographic inks, paints, and enamels.
Figure 2.1 Atomic structural models of (a) graphite and (b) carbon black
(Baker et al., 1992)
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Table 2.1 Chemical composition of carbon blacks, % (Baker et al., 1992).
Type
Rubber-grade
Medium thermal
Acetylene
furnace
Carbon
97.3-99.3
99.4
99.8
Hydrogen
0.20-0.40
0.30-0.50
0.05-0.10
Oxygen
0.20-1.20
0.00-0.12
0.10-0.15
Sulfur
0.20-1.20
0.00-0.25
0.02-0.05
Ash
0.10-1.00
0.20-0.38
0.00
Volatile
0.60-1.50
<0.40
2.2 Paper Fiber
Paper means a thin layer object, which is made from fiber and mixed
with more than one type of additives (Watanawong, 2000).
2.2.1 Source of Paper Fiber
2.2.1.1 Wood
 Softwood is coniferous or gymnosperm species such
as spruce, pine, fir.
The trade name of paper
normally has N (needle) at the initial name, for
example, NBKP (Needle Bleached Kraft Pulp)
 Hardwood is angiosper species such as Eucalyptus,
birch, aspen. The trade name of paper normally has L
(leaved) at the initial name, for example, LBKP
(Leaved Bleached Kraft Pulp).
2.2.1.2 Non-Wood
 Agriculture residue such as millet, bagasse
 Natural growing plants such as bamboo
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 Crop fibers - Paper fiber is produced from bast or
stem such as hemp, ramic, sute, kenaf.
- Paper fiber is produced from leaf fiber
such as abaca, sisal, and pineapple.
- Paper fiber is produced from seed fiber
such as cotton
2.2.2 Chemical Composition of Wood
Chemical composition of some variety of sources fiber
composes of cellulose, hemicellulose, lignin and extractive.
2.2.2.1 Cellulose
It is a straight chain homopolymer of d-glucose, which
is connected with 1,4  glucosidic bond. It comprises of 10,000 units and Hbond link between row.
2.2.2.2 Hemicellulose
It is heteropolymer which is combined approximately
200 units of many types of sugars such as glucose, mannose, xylose and
arabinose.
2.2.2.3 Lignin
It is amorphous, which has approximately 2,500 units of
phenyl propane units. The function of lignin is the binder between fibers.
2.2.2.4 Extractive
It is the compound, which can be soluble in organic
solvent, for instance, acetone, alcohol, dicholomethane and chloroform.
m
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2.2.3 Composition of Paper Fiber
2.2.3.1 Fibrous Materials
Generally, paper composes of fiber in 70 to 95 percent
of paperweight. The quality of paper depends on quantity of fiber. Fibers
comprise of plant cells such as parenchyma cell, vessel cell.
2.2.3.2 Non Fibrous Materials
They are additives, which are added for improving the
quality of paper. They are classified into 2 parts.
 Functional additive is added to improve the specific
property of paper. It can be divided into 6 types as
following.
 Sizing agent is the chemical which increase the
tolerance of water. Paper which contains no added
sizing agent is easy for adsorbing water such as
tissue paper, blotting paper. Sizing agents use in
paper process are alum/rosin size, wax, asphalt,
AKD (Alkyl Ketone Dimer) and ASA (Alkenyl
Succinic Anhydride).
 Filler is added in order to increase surface area of
paper. It also improves the properties of paper
such as increase in ore particle-air and ore
particle-fiber,
light
scattering,
smoothness,
brightness and decrease in budget of paper
production. The fillers use in paper process are
kaolin or clay, TiO2 (Titanium dioxide) and
CaCO3 (Calcium carbonate)
 Dry strength agent is added for increasing
toughness of paper. The agent which is commonly
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used in paper process is cationic starch and
polyacrylamide.
 Wet strength agent is added for increasing
toughness at 15 percent of originally toughness.
They
are,
for
instance,
melamine-formaldehyde,
urea-formaldehyde,
polyamide
and
polyamine
 Dyes is added for colouring paper, tinting dyes.
 Optical brightenings agent is fluorescent dyes that
adsorbs UV ultraviolet and desorbs wavelength in
range of blue.
 Chemical processing aids
 Retention aids
 Deformers help preventing foam formation.
 Microbiological control agent prevent the slime
formation.
 Pitch control agent
 Drainage aids increase the rate of leaving water
from paper.
 Formation aids decrease the aggregate of fiber.
2.3 General Structure of Surfactants
The term surfactant is a contraction of surface-active agent which has
a characteristic molecule consisting of groups of opposing solubility
tendencies, typically an oil-soluble hydrocarbon chain, known as a
hydrophobic group, and a water-soluble ionic group, called hydrophilic group.
This is known as an amphipathic structure.
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Depending on the nature of the hydrophilic group, surfactants are
classified as:
2.3.1 Anionic
The surface-active portion of the molecule bears negative
charge, for example, RCOO-Na+ (soap) RC6H4SO3-Na+ (LAS or linear alkyl
benzene sulfate).
2.3.2 Cationic
The surface-active portion of the molecule bears positive
charge,
for
example,
CH3(CH2)15N+(CH3)3Br-
(CTAB
or
Cetyl
trimethylammonium bromide), RNH3+Cl- (salt of a long chain amine) RN(CH3)3+Cl- (quarternary ammonium chloride).
2.3.3 Zwitterionic
The surface-active portion of the molecule bears both positive
and negative charges, for example, RN+H2CH2COO- (long-chain amino acid)
RN+(CH3)2 (sulfobetaine).
2.3.4 Nonionic
The surface-active portion of the molecule bears nonionic
charge, for example, RCOOCH2CHOHCH2OH (monoglyceride of long-chain
fatty acid), RC6H4(OC2H4)xOH (polyoxyethylenated alkylphenol).
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2.4 Roles of Surfactant on Flotation Deinking
According to Ferguson (1992b), surfactants used in flotation deinking
take three important roles.
 Acting as a dispersant to separate ink particles from the fiber and
prevent redeposition of separated ink particles on the fiber.
 Acting as a collector to agglomerate the small ink particles to floc
by controlling the surface of ink into hydrophobic surface.
 Acting as a frother to generate foam on the top layer of flotation
cell for ink removal.
2.5 Adsorption Isotherm of Surfactant
The adsorption isotherm for a monoisomeric surfactant was first
appeared in the work of Somasundaran and Fuerstenau in 1966.
The
schematic diagram of a typical adsorption isotherm for monoisomeric
surfactant was illustrated in Figure 2.2 (Harwell and Scamehorn, 1993). The
adsorption isotherm was divided into three (or four) distinct regions as
follows:
Region I is commonly referred to as the Henry’s Law region because
in this region monoisomeric surfactant isotherms are linear and have a slope of
unity.
In the Henry’s law region, surfactant adsorption is the result of
monomer interactions with the surface.
There is little or no interaction
between individual adsorbed surface ions.
Region II is characterized by a sharply increased isotherm slope
relative to the slope in the Henry’s Law region. This is a general indication of
the onset of cooperative effects between adsorbed molecules.
12
1000
Equilibrium surfactant adsorption
(micromoles/g)
Region I
Region II
Region III
Region IV
100
10
1
0.1
0.1
1
10
100
Equilibrium surfactant concentration (mM)
Figure 2.2 Typical surfactant adsorption isotherm.
It is widely accepted that this cooperativity consists of formation of micellelike aggregates of adsorbed surfactants.
These aggregates are frequently
called admicelles or hemimicelles, depending on whether their morphology is
viewed as local bilayers or local monolayers and the transition point from
Region I to Region II is called the critical admicelle concentration (CAC) or
hemimicelle concentration (HMC). As the driving force for micelle formation
is the tail-tail interactions in the micelles, so for admicelles and hemimicelles
their formation is driven by hydrophobic interactions between tail groups.
Scamehorn et al., (1982) demonstrated that hemimicelles first formed on the
most energetic surface sites. Cases and Vilieras, (1992) have represented
evidence to indicate that the reason for the formation of these aggregates
locally or patchwise at the interface is due to the heterogeneity of the surface.
Region III is characterized by a decrease in the isotherm slope
relative to the slope in Region II, the change in slope may be abrupt, as in the
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schematic, or it may be gradual. An explanation for this change in slope is
that with increasing adsorption of surfactants, the surface becomes likecharged to the surfactant and the surface begins to repel the surfactant ions.
However, this mechanism cannot be the explanation for the same isotherm
shape for nonionic surfactant adsorption.
Region IV is the plateau adsorption region for surfactants. Generally,
the Region III/Region IV transition occurs approximately at the CMC of the
surfactant, and reflects the effect of micelle formation on the chemical
potential of surfactant monomers, just as the formation of micelles affects the
variation of surface tension with surfactant concentration. In some systems,
however, the Region III/Region IV transition can be reached when the surface
becomes saturated with adsorbed surfactant. For the adsorption of surfactants
from aqueous solutions, this will correspond to bilayer completion for ionic
surfactants adsorbed on oppositely charged surfaces, or to monolayer
completion for adsorption on hydrophobic surfaces.
2.6 Roles of Calcium Ions in Flotation Deinking
The collector chemistry implies the use of anionic surfactant and a
“collector” ion, such as calcium, to impart the desired properties to the ink
particles. The fundamental aspects and the roles of calcium ions in flotation
deinking have been proposed.
Larsson et al. (1984) studied the zeta potential and flotation efficiency
as a function of the addition of calcium ion concentration to model ink
dispersions in the presence of sodium stearate. They verified that the absolute
value of the negative zeta potential of the ink particles decreased with
increasing calcium concentration while flotation efficiency increased. It was
suggested that, in alkaline medium, generally adopted industrially, the highly
negative charge of the ink particle promoted its electrostatic stabilization.
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This stabilization facilitated the liberation of the ink particle from the fiber.
Nevertheless, this charge prevented the adhesion of ink particles on the air
bubbles.
It was observed that when calcium chloride was added to the
flotation pulp, the salt of calcium/collector was partially precipitated on the
surface of ink particles and hence became less negatively charged.
The
precipitation of calcium dicarboxylate was necessary to create a microencapsulation of the ink particles causing aggregation, hydrophobicity, and
subsequent flotation.
There are a few problems occurring in flotation deinking and leading
to less efficiency, for example, fiber loss. A major cause of fiber loss is the
presence of calcium ion in the deinking system. A hypothesis to explain how
fibers float was reported by Turvey (1993). He suggested that calcium ions
form a complex with print components which sticks to fibers and gives them a
hydrophobic surface, the fiber will have an increasing tendency to float during
flotation deinking.
In addition, Dorris and Ngugen (1995) studied flotation of model inks
(flexographic inks) without fibers. Flexographic inks which were composed
of a pigment, usually carbon black that dispersed in water with the use of
water soluble carboxylic derivatives, which also performed as binder. The
results showed that, in basic condition, the floatability of flexographic inks at
alkaline condition improved with the addition with calcium ions and much
more so with the addition of both sodium oleate and calcium ions, under
turbulent agitation. The results suggested that conventional calcium soaps of
fatty acids were effective flotation collectors for flexographic inks. Contrary
results obtained in flotation deinking mills suggest that the problem of
deinkability of flexo inks may not be due only to poor floatability, but also to
other phenomena such as ink redeposition on fibers.
Oliveira and Torem (1996) investigated the fundamentals of ink
flotation with sodium stearate, oleic acid and sodium dodecyl sulfate
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collectors as well as the influence of metallic cations such as Ca2+, Mg2+and
Al3+ on floatability.
The results indicated that, with sodium stearate, an
increase in floatability was probably associated with the precipitation of
stearate species on the ink surface.
It was suggested that a non-specific
adsorption mechanism had occurred with sodium dodecyl sulphate, whilst a
specific adsorption mechanism was associated with oleic acid. In the presence
of calcium cation, an increase in floatability was verified, probably due to
calcium/collector salt precipitation on the ink particle surface. In the presence
of magnesium and aluminum species, an increase in floatability of ink was
noted for pH ranges related to the concentration of hydroxy complexes and to
the predominance of metallic hydroxides at pH values below their the
isoelectric points (IEPs).
In the above references, the mechanism on the floatability of ink
particles was supported by the precipitation mechanism. Conditions under
which no surfactant precipitated was presented were not considered.
Rutland and Pugh (1997) studied mechanism of surfactant and
calcium in adsorption by surface force and coagulation technique. The surface
force technique concerned the invention of fatty acid flotation collectors and
calcium activator with a negatively charged mica substrate at high pH. Since
the surface of ink particles under deinking conditions are enriched by
negatively charged groups, these experiments enabled some details of the
fundamental mechanisms involved in deinking flotation to be clarified. The
results (carried out at relatively low calcium and fatty acid concentrations) are
summarized as follows: (a) At pH greater than 10, the negatively charged
surface generated a long range DLVO double-layer repulsion and the potential
at the mica/electrolyte interface could be estimated.
However, at short
distances, a repulsive non-DLVO hydration barrier was detected due to
adsorbed Na+. (b) On addition of CaCl2, the Na+ was replaced by the specific
adsorption of the less strongly hydrated Ca(OH)+ solution species or more
16
strongly hydrolyzable calcium species. This resulted in the elimination of the
hydration forces and mica-mica contact. On addition of fatty acid, no change
in the force profile was detected, suggesting no calcium was removed from
the surface and there was no evidence of calcium soap formation in the surface
region. This result implies that under alkali conditions, the calcium does not
induce a bridging mechanism in the presence of fatty acid (below the calcium
soap precipitation level).
In fact, the calcium could be only operated as
bridging agents if they can specifically bind to the surface, as well as to the
carboxylated
fatty
acid.
The
“calcium
dehydration/destabilization
mechanism” was verified by coagulation studies with quartz suspension. At
higher concentrations of fatty acid and calcium, calcium soap was precipitated
in bulk solution.
It was suggested that microencapsulation of
the ink
particles with hydrophobic species occurs through heterocoagulation with the
bulk precipitated calcium soap particles.
Rivello et al. (1997) studied ink removal from secondary fiber by
flotation deinking. The underlying mechanism of “ collector chemistry ” are
elucidated and found to be complex combinations of the ink particle
interactions stemming from the nature of the surfactant/calcium relationship.
This study was focused on two medium chain-length fatty acids (sodium
octanoate and sodium dodecanoate) and a comparable chain-length sulfate
(sodium dodecyl sulfate). The system behavior are observed below the Ksp of
the surfactant/calcium complex by investigating the adsorption isotherms of
the surfactants on both model inks and fibers, the associated zeta potentials,
the aggregation characteristics of the model ink and the flotation of the ink and
fibers. The fundamental mechanisms were found to be adsorptive rather than
precipitative. It was proposed that the surfactant adsorbed with a tail-down
and/or lying down geometry on the carbon surfaces and the calcium strongly
associated with the anionic head groups without bulk phase precipitation. The
specific interactions between the carboxylate, the Ca2+ and the fiber result in
17
decreased calcium adsorption with increasing C8 concentration (a “calcium
exclusion” effect). The interaction between paper fiber and calcium ions was
purely electrostatic and non-associative adsorption. The calcium ions were
attracted by the negatively charged sites on paper surface resulting from
ionization of carboxyl and hydroxyl group.
2.7 The Double Layer
The diffuse layer can be visualized as a charged atmosphere
surrounding the colloid as shown in Figure 2.3 (Adamson, 1990). The double
layer model is used to visualize the ionic environment in the vicinity of a
charged colloid and explains how electrical repulsive forces occur. The effect
of the colloid on the positive ions (often called counter-ions) in solution was
studied. Initially, attraction from the negative colloid causes some of the
positive ions to form a firmly attached layer around the surface of the colloid;
this layer of counter-ions is known as the stern layer.
Additional positive ions are still attracted by the negative colloid, but
at the same time they are repelled by the stern layer as well as other positive
ions trying to approach the colloid. This dynamic equilibrium results in the
formation of a diffuse layer of counter ions. They have a high concentration
near the surface, which gradually decreases with distance, until it reaches
equilibrium with the counter-ion concentration in the solution.
Similarly, but opposite fashion, there is a lack of negative ions in the
neighborhood of the surface because they are repelled by the negative colloid.
Negative ions are called co-ions because they have the same charge as the
colloid.
Their concentration will gradually increase with distance, as the
repulsive forces of the colloid are screened out by the positive ions, until
equilibrium is again reached.
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The charge density at any distance from the surface is equal to the
difference in concentration of positive and negative ions at that point. Charge
density is greatest near the colloid and gradually diminishes toward zero as the
concentration of positive and negative ions merge together. The attached
counter-ions in the stern layer and the diffuse layer are what we refer to as the
double layer.
The thickness of this layer depends upon the type and
concentration of ions in solution.
Figure 2.3 Visualization of the double layer.
2.8 Zeta Potential
The double layer is formed in order to neutralize the charged colloid
and, in turn, causes an electrokinetic potential between the surface of the
colloid and any points in the mass of the suspending liquid. This voltage
19
difference is on the order of millivolts and referred to as the surface potential.
Figure 2.4 shows the magnitude of the surface potential through the distance
from colloid. It is related to the surface charge and the thickness of the double
layer. As we leave the surface, the potential drops off roughly linearly in the
stern layer and then exponentially through the diffuse layer, approaching zero
at the imaginary boundary of the double layer. The potential curve is useful
because it indicates the strength of the electrical force between at which this
force comes into play.
A charged particle will be moved with a fixed velocity in a voltage
field. This phenomenon is called electrophoresis. The particle’s mobility is
related to the dielectric constant and viscosity of the suspending liquid and to
the electrical potential at the boundary between the moving particle and liquid.
This boundary is called the slip plan and is usually defined as the point where
the stern layer and the diffuse layer meet. The stern layer is considered to be
rigidly attached to the colloid, while the diffuse layer is not. As a result, the
electrical potential at this junction is related to the mobility of the particle and
is called the zeta potential.
Figure 2.4 Relationship between zeta potential and surface potential.